Wood does not rot as fast as that!
On the other hand it does not last for thousands of years. Polystrate fossils are found going through layers that are said in evolution mythology to be laid down over hundreds of thousands of years. That is not credible. At the very least one would expect the tops of the trees to be significantly damaged or reduced through their longer exposure, which is not generally seen, I believe. Just look around the world and try to find an example of surviving trees that are being gradually buried after being dead for centuries. I do not think you will find a single one.
The mechanism that has broken and displaced those trees from Mt St Helens is the same one that will have operated on a vastly larger scale during the flood. With millions of cubic kilometres of sediment being rapidly deposited, these trees will have been rapidly buried, while the water created the layers surrounding them. At Mt St Helens, layered deposits many metres thick are known to have been laid down in a matter of hours or days.
There is no proof that the polystrate fossils were dead for centuries before being buried. That is an unprovable assumption. Spirit lake is an example of the process happening at a much slower rate than suggested possible, yet it is happening.
The second part of your argument has major flaws. Sediment of that magnitude would have covered every bit of plant life, so not only should there be polystrate trees, but bushes, vines, flowers as well and with every tree made into a polystrate fossil, you should be able to dig in your back yard and find quite a few, but you can't
Of course, even if they weren't all encased in sediment, being underwater for that length of time would kill all the plant life thus cutting off all oxygen manufacture for the planet, not to mention the entire food supply for all herbivores.
There's also the fact that polystrate fossils go through different layers of different types of sediment, but the flood would have mixed everything together and made one type of sediment, not many.